To solder semiconductor devices according to a circuit pattern of electrical conductors on a printed circuit board, the semiconductor devices must be first placed in their correct soldering position.
Conventionally, this positioning has been implemented by visually checking the relative position of the leads of the semiconductor devices and the circuit pattern of the printed circuit board. However, the accuracy and efficiency of this visual method have decreased with an increase in the density of circuit integration and a resulting decrease in the lead pitch.
The solutions for this problem disclosed in U.S. Pat. Nos. 5,060,288 and 5,235,407, for example, pick up an image of semiconductor devices just before they are aligned to the circuit pattern of a printed circuit board on to which they are to be soldered. The obtained picture is enlarged on a display for checking whether they are properly aligned or not.
These methods check the accuracy of alignment based on images taken from above the semiconductor devices to be mounted. Therefore, they are too some extent effective for semiconductor devices of the type whose leads project outward. However, they are incapable of checking the alignment of the leads of semiconductor devices of the PLCC package type whose leads are provided on the back thereof, as shown at (a) in FIG. 10, with the circuit pattern on printed circuit boards. This problem is solved by marking points for aligning semiconductor devices on a printed circuit board to permit indirect confirmation of their alignment. However, this method is not without problems with reliability.
To overcome these shortcomings of the conventional methods, the inventor proposed a positioner that brings the leads of semiconductor devices and a circuit pattern into proper alignment by using a composite image thereof formed by means of a semitransparent mirror interposed between the devices and a substrate on to which they are to be mounted, with the lead terminals and the circuit pattern thereof disposed to face each other, as disclosed in Japanese Provisional Patent Publication No. 171998 of 1992.
This positioner permits accurate positioning as the position of the semiconductor devices and circuit pattern is directly confirmed.
With the recent rapid development of integrated circuits, not only the size of semiconductor devices but also the density of lead pin arrangement has increased. Accurate positioning of such complex devices necessitates higher magnifications than can be obtained by increasing the size of the semitransparent mirror. However, a larger semitransparent mirror requires a correspondingly larger camera and other optical devices that may, in combination, not be operated as efficiently as smaller ones.